The general problem of ribosome production in eukaryotic cells has been studied. One line of rat myoblasts differentiates and decreases ribosome production. Our findings show that differentiation is associated with uncoordinate synthesis. R-RNA transcription is decreased 5-10 fold while r-protein synthesis is the same as control. A temperature sensitive BHK line synthesizes r-RNA and protein. Assembly in the nucleolus occurs, but complete procesing is interrupted at the non-permissive temperature.